Abstract: Frontline investigations with fighters against the Islamic State (ISIL or ISIS), combined with multiple online studies, address willingness to fight and die in intergroup conflict. The general focus is on non-utilitarian aspects of human conflict, which combatants themselves deem ‘sacred’ or ‘spiritual’, whether secular or religious. Here we investigate two key components of a theoretical framework we call ‘the devoted actor’ — sacred values and identity fusion with a group­ — to better understand people’s willingness to make costly sacrifices. We reveal three crucial factors: commitment to non-negotiable sacred values and the groups that the actors are wholly fused with; readiness to forsake kin for those values; and perceived spiritual strength of ingroup versus foes as more important than relative material strength. We directly relate expressed willingness for action to behaviour as a check on claims that decisions in extreme conflicts are driven by cost-benefit calculations, which may help to inform policy decisions for the common defense.